Beaches of the Gulf Coast

1. Coastal Processes

3 1 Coastal Processes W HEN we hear the word beach, the first thing that comes to mind is sand; the next is probably waves. Actually there are multiple processes that impact beaches and control their existence and appearance (figure 1.1). It is appropriate to begin with the most fundamental of these coastal processes: the weather. Then it is important to consider how the waves, which are a result of the weather, impact the beach. These waves also generate currents that are a major element of beach dynamics. Storms, especially hurricanes, are a significant factor in Gulf of Mexico beaches. A process that is always present but is not weather related is the ebb and flow of tides, but tides do not play a major role in Gulf Coast beaches. Weather The Gulf Coast is positioned in the latitudes that range from about 18° to 30° north of the equator.This range of latitudes experiences a fairly wide variation in weather patterns. As the seasons change, so do the weather patterns. During the summer the Gulf is within theTrade Winds belt, with the prevailing direction from the southeast. This is the time when tropical storms can impact this coast. In the winter the westerlies prevail as weather systems are moving from the northwest to the southeast. The changes from one pattern to another influence the way beaches respond to the wind and the waves produced by it. Westerlies In the midlatitudes, the weather typically moves from west to east. Highpressure systems tend to come from the higher latitudes and collide with low- 4 General CharaCTerisTiCs and dynaMiCs of BeaChes pressure areas, causing precipitation. It should be noted that wind tends to flow from high pressure toward low pressure. As these systems move across the mainland of the United States, they dominate the weather. For much of the year, this type of weather does not influence the Gulf Coast, but in the winter these patterns move into more southern latitudes. For example, the Florida peninsular coast experiences prevailing wind from the southeast from about mid-March to mid-October. As the seasons change, the sun moves to the southern hemisphere and the frontal systems of westerly weather impact this peninsula. The typical situation occurs when a cold front (high pressure) comes from the northwest and passes across the Texas coast (figure 1.2a). As the front approaches , the wind is blowing onto the coast from the southeast. Just behind the front the wind is from the northwest and is typically strong. Lower temperatures and dryer air are associated with such frontal passages. On the Texas coast intense frontal passages are commonly called “blue northers” because of the cold north wind associated with them. These frontal systems move across the Gulf of Mexico toward the Florida peninsula. There is some loss of intensity due to the warming effect of the Gulf Figure 1.1. Various processes and other variables that constitute the beach system. 5 CoasTal ProCesses Figure 1.2. Frontal system as it passes (a) offshore of Texas and moves (b) onshore on the Florida coast; (a) (b) 6 General CharaCTerisTiCs and dynaMiCs of BeaChes (c) general simulation model for coastal processes. From R. A. Davis and W. T. Fox, “Process-Response Patterns in Beach and Nearshore Sedimentation: I. Mustang Island, Texas,” Journal of Sedimentary Research 45 (1975): 852–65. 7 CoasTal ProCesses water. As the front approaches the Florida coast, the wind has a southerly component . There is a rapid change to the northwest with increased speed as the front passes (figure 1.2b). Under some circumstances these fronts pass across the Florida peninsula and may move northerly along the Atlantic Coast to become a nor’easter, the strong storms of the winter in the New England area. This pattern continues for several months, with the intensity of each frontal system increasing as the winter weather gets colder. Such frontal passages tend to dominate the wave and current patterns along the Gulf, especially on the Texas and Florida coasts (figure 1.2c). Tropical Storms and Hurricanes During the warm months, intense storms that develop in the tropical eastern Atlantic can impact the Gulf Coast.These storms begin as so-called tropical waves off the west coast of Africa. As the system gains intensity, it becomes a circulating low-pressure system. Because of its position in the low latitudes, it moves westward as part of theTrade Winds system...